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New Lepton Universality Measurements Show Agreement with Standard Model

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Physicists from the LHCb Collaboration at CERN’s Large Hadron Collider (LHC) have performed new tests of lepton universality, one of the basic principles of the Standard Model of particle physics. This principle states that the Standard Model treats the three charged leptons (electrons, muons and taus) identically, except for differences due to their different masses. The new results, published in two papers on, are in agreement with the Standard Model.

View of the LHCb detector. Image credit: CERN.

A central mystery of particle physics is why the 12 elementary quarks and leptons are arranged in pairs across three generations that are identical in all but mass, with ordinary matter comprising particles from the first, lightest generation.

Lepton flavor universality states that the fundamental forces are blind to the generation to which a lepton belongs.

In recent years, however, an accumulation of results from LHCb and experiments in Japan and the United States have suggested that this might not be the case, generating cautious excitement among physicists that a more fundamental theory — perhaps one that sheds light on the Standard Model’s mysterious flavor structure — might reveal itself at the LHC.

Interest in the ‘flavor anomalies’ peaked in March 2021, when LHCb presented new results comparing the rates at which certain B mesons, composite particles that contain beauty quarks, decay into muons and electrons.

According to the theory, decays involving muons and electrons should occur at the same rate, once differences in the leptons’ masses are accounted for.

But the LHCb results hinted that B mesons decay into muons at a lower rate than predicted, as indicated by the results’ statistical significance of 3.1 standard deviations from the Standard Model prediction.

The new LHCb analysis, which has been ongoing for the past five years, is more comprehensive.

It considers two different B-meson decay modes simultaneously for the first time and provides better control of the background processes that can mimic the decays of B-mesons to electrons.

In addition, the two decay modes are measured in two different mass regions, thus yielding four independent comparisons of the decays.

The results, which supersede previous comparisons, are in excellent agreement with the principle of lepton flavor universality.

“Measurements of the ratios of rare B-meson decays to electrons and muons have generated much interest in recent years because they are theoretically ‘clean’ and show consistency with a pattern of anomalies seen in other flavor processes,” said LHCb Collaboration spokesperson Professor Chris Parkes, a physicist at the University of Manchester and CERN.

“The results shown today are the product of a comprehensive study of the two main modes using our full data sample and applying new, more robust techniques.”

“These results are compatible with the expectation of our theory.”


LHCb Collaboration. 2022. Test of lepton universality in b→sℓ+ℓ− decays. arXiv: 2212.09152

LHCb Collaboration. 2022. Measurement of lepton universality parameters in B+→K+ℓ+ℓ− and B0→K*0ℓ+ℓ− decays. arXiv: 2212.09153

Source : Breaking Science News

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